In the semiconductor industry, it is desirable to obtain temperature uniformity in the substrate during temperature cycling of substrate. Temperature uniformity provides uniform process variables on the substrate (e.g. layer thickness, resistivity, etch depth) for temperature activated steps such as film deposition, oxide growth and etching. In addition, temperature uniformity in the substrate is necessary to prevent thermal stress-induced wafer damage such as warpage, defect generation and slip.
FIG. 1 schematically shows a prior art flood type rapid thermal heating apparatus in which a wafer 11 disposed in chamber 12 is heated by radiation from a plurality of lamps 13. This type of heating apparatus does not provide adequate spatial control of temperature. The primary difficulty is that different areas on the wafer may have different energy absorption or emissivity characteristics. For example, if a flood heating source (assuming uniform illumination across the wafer) is used to heat a wafer during a rapid thermal processing cycle in which the thermally-isolated wafer may be ramped in temperature on the order of 10-300xc2x0 C./sec, the edge will maintain a different temperature than the middle because the edge can accept radiant energy from or emit radiant energy to a wider field of view. FIG. 2 shows the temperature at the center and edges of a wafer as a function of time for a flood type heating source During the temperature ramp-up portions of the heating cycle the edges will be hotter than the center while during the steady state and ramped-down portions the edges will be cooler than the center. These edge to center temperature differences create radial stresses in a wafer which if large enough, can damage the wafer, and are not tolerable in many processes, especially high temperature processes in which the mechanical strength of the wafer is substantially reduced. For example, at 1150xc2x0 C. the center to edge temperature difference on a four inch silicon wafer of approximately 5xc2x0 C. can induce dislocation formation and slip. Some conventional flood heating sources, such as a bank of tungsten-halogen lamps or a single arc lamp, can be modified to compensate for center-to-edge temperature differences of one sign, for example, during the temperature ramp up. Shading or reflectors can be used to provide more light energy to the center of the wafer as compared to the edge, but it is impossible for such a heating source to provide temperature uniformity during all parts of the thermal cycle.
Temperature gradients can also be induced by other sources. For example, a wafer may have non-uniform emissivity because of spatial modifications to surface areas or volumes of the wafer. These modifications could include films which have been patterned by photolithography or locally doped regions such as buried layers for bipolar transistors. In addition, the temperature gradients can be induced by localized gas cooling or heating effects, as well as non-uniform endothermic or exothermic reactions which may occur on the substrate surface during processing.
It is a general object of this invention to provide an improved heat source for rapid thermal processing of semiconductor wafers or substrates.
It is a further object of this invention to provide a heat source which allows spatial control of the heat energy applied to the substrate.
It is another object of this invention to provide a heat source which enables spatial control of the heat applied to a wafer or substrate to maintain temperature uniformity despite localized variations in the ability of the wafer to emit or absorb heat energy.
It is another object of this invention to provide a heat source which includes a plurality of independently controlled heat sources which provide energy to predetermined overlapping areas of a substrate or wafer.
It is a further object of this invention to provide a heat source including a plurality of independently controlled heat sources each providing energy to a predetermined area of a substrate or wafer and sensors for sensing the temperature of said area to permit control of applied energy to maintain a uniform temperature across the wafer.
It is still a further object of this invention to provide a heat source which includes a plurality of light pipes, each of which directs energy from an energy source onto predetermined overlapping areas of a wafer or substrate.
It is a further object of this invention to provide an improved light pipe heat source and a process chamber window assembly.
The foregoing and other objects of this invention are achieved by a heat source which includes a plurality of sources of radiant energy each serving to radiate a predetermined area of the substrate, and means for mounting the sources of radiant energy next to each other so that portions of the radiated areas of adjacent sources overlap and the energy intensity at said portions from the different sources add, and a control means for controlling the intensity of each of said sources of radiant energy whereby to control the intensity of radiation at different areas on said wafer or substrate.
More particularly, the invention includes a plurality of radiant energy sources each associated with a light pipe mounted next to another light pipe with the light pipes serving to direct radiant energy from the associated source towards the substrate to radiate a predetermined area of the substrate with a pattern of relative radiant intensity. The light pipes are spaced so that a portion of the radiated area of adjacent light pipes overlaps so that the intensity of radiation at said portions adds to provide a relative intensity across the wafer which is dependent upon the intensity of a combination of said radiant energy sources.